深入理解ajax同步和异步的区别

同步可以改变外部定义的变量值，异步可以提高加载效率

<script>
var a = 0
function loadDoc(file,async=true){
if(window.XMLHttpRequest){  // code for IE7+, Firefox, Chrome, Opera, Safari
    xmlhttp=new XMLHttpRequest();
}else{  // code for IE6, IE5
    xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
if(async === true){
    xmlhttp.onreadystatechange = function(){
        if(xmlhttp.readyState < 4){
            // 加载中
        }else if (xmlhttp.readyState == 4 && xmlhttp.status == 200){
            // 成功
            xmlDoc=xmlhttp.responseText;
            a = 1
            return xmlDoc
        }else{
            // 失败
            xmlhttp.abort()
            return
        }
    }
}
xmlhttp.open("GET",file,async);
xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xmlhttp.send();
if(async === false){
    xmlDoc=xmlhttp.responseText;
    a = 1
    return xmlDoc
}
}
var ret = loadDoc('http://localhost/test.php',true)
console.log(a)
var ret = loadDoc('http://localhost/test.php',false)
console.log(a)
</script>